@rockulius could your duck have somehow injured it's bill stuck it in fencing or some thing similar I'd use some kind of antibiotic salve or cream on it. Or Veterycin wound care spray. you can also add some Apple Cider Vinegar to the drinking water which can help to heal 1-2 Tablespoons to 1 gal of fresh water. use this solution daily until you see noticeable healing then cut back to a couple times a week. it is also a good tonic for out flocks. If it look like an infection or a fungus then I'd think about taking your duck to a vet.

Can you post a picture? My Pekin has black spots, but it's a normal hormone response. Crusty is not normal, though. It would be great if we could see what you are talking about. @Miss Lydia already gave some great advice on how to care for it.

Can you post a picture? My Pekin has black spots, but it's a normal hormone response. Crusty is not normal, though. It would be great if we could see what you are talking about. @Miss Lydia already gave some great advice on how to care for it.